1. Field of the Invention
The present invention relates to a Group III nitride-based compound semiconductor device having a characteristic shape of an n-type region where a negative electrode has been provided. As used herein, the term “Group III nitride-based compound semiconductor” encompasses a semiconductor represented by the formula AlxGayIn1-x-yN (0≦x≦1, 0≦y≦1, 0≦x+y≦1); such a semiconductor containing a predetermined element so as to attain, for example, an n-type/p-type conduction; and such a semiconductor in which a portion of a Group III element is substituted by B or Tl, and a portion of the Group V element is substituted by P, As, Sb, or Bi.
2. Background Art
In the development of AlxGayIn1-x-yN semiconductors, crystallinity of the semiconductor has been enhanced, and conductivity thereof has been well-controlled, leading to production of a variety of semiconductor devices such as light-emitting diodes, laser diodes, and HEMTs. Currently, in a typical production process for semiconductor devices, a plurality of Group III nitride-based compound semiconductor layers are formed through epitaxial growth on a C-plane or A-plane sapphire substrate, silicon substrate, or SiC substrate. In many cases, when such Group III nitride-based compound semiconductor epitaxial layers are formed on the aforementioned hetero-substrate, the growth surface of epitaxial growth is a C-plane such that the thickness of each Group III nitride-based compound semiconductor layer increases along the c-axis. Also, when epitaxial growth is performed on a GaN thick layer having a C-plane as a main plane and serving as a growth substrate, the growth surface of an epitaxial is a C-plane such that the thickness of the epitaxial film increases along the c-axis.
Meanwhile, when the thickness direction of a GaN substrate is the c-axis direction, one surface normal to the c-axis is a Ga-polar surface, and the other surface is an N-polar surface. The Ga-polarity is the direction of the vector from the Ga atom to the nitrogen atom between Ga and nitrogen atoms which are connected with each other parallel to c-axis. Whereas the nitrogen-polarity is the direction of the vector from the nitrogen atom to the Ga atom between nitrogen and Ga atoms which are connected with each other parallel to c-axis. Accordingly, Ga-polar surface is defined as the surface whose normal vector oriented to the outside is equal to the Ga-polarity. Whereas nitrogen-polar surface is defined as the surface whose normal vector oriented to the outside is equal to the nitrogen-polarity.
When GaN is epitaxially grown on the aforementioned hetero-substrate such that the C-plane is a growth surface, the uppermost layer of the epitaxial film assumes a Ga-polar surface. When a C-plane GaN substrate is employed and GaN is epitaxially grown on the Ga-polar surface, the uppermost layer of the epitaxial film assumes a Ga-polar surface. However, those skilled in the art know that when epitaxial growth is performed on the N-polar surface of the GaN substrate through MOVPE or halide VPE, an epitaxial film having high crystallinity cannot be formed.
In the case where the thickness direction of a Group III nitride-based compound semiconductor layer is the c-axis direction and a plurality of such semiconductor layers are stacked, the interface between two layers assumes C-plane. In this case, the following problem is known to arise in an HEMT or a light-emitting device having an MQW light-emitting layer.
In an HEMT having a two-dimensional electron gas layer provided between an InAlGaN layer and an n−-type GaN layer, when the polarization of the InAlGaN layer is greater than that of GaN and each interlayer interface assumes C-plane, electrons tend to be accumulated in a hetero-interface. Thus, in this case, device characteristics of interest may fail to be attained. For example, normally-off cannot be attained.
When a Group III nitride-based compound semiconductor is formed on a sapphire substrate having R-plane ((1-102) plane) as a main plane such that A-plane ((11-20) plane); i.e., a non-polar surface, of the semiconductor serves as a growth surface, a polarization field perpendicular to a hetero-interface is not formed. In this case, generally, a normally-off transistor can be produced, and electrons can run without being affected by a polarization field, which is advantageous in high-speed operation.
In relation to light-emitting devices, there has been proposed a so-called type-II quantum well active layer, which is an MQW light-emitting layer (active layer) in which electrons are quantumized and confined in one layer, and holes in another layer. When a type-II quantum well active layer is provided on a non-polar surface such as non-polar A-plane ((11-20) plane), the active layer is not affected by polarization, and high light emission efficiency is realized.
Thus, recently, a A-plane GaN substrate or a M-plane GaN substrate have been commercialized, and devices in which a Group III nitride-based compound semiconductor layer having a thickness direction of the a-axis or m-axis (i.e., non-polar axis) direction is provided on such a GaN substrate have become of interest.
Japanese Patent Application Laid-Open (kokai) No. 2007-43164 discloses that an electrode is difficult to form on the N-polar surface of an n-type GaN substrate, which is opposite the Ga-polar surface on which a device has been provided.